Growth hormone (GH) is a pituitary-derived peptide hormone with potent somatogenic and metabolic actions. GH binds its receptor (GHR), a cell surface transmembrane protein enriched in liver, muscle, fat, and heart to exert profound effects on fuel metabolism, muscle mass, and energy homeostasis in humans and other vertebrates. GH sensitivity is altered in the setting of acute and chronic disease states and it is uncertain whether enhancing or inhibiting GH action would be salutary in such situations; this is important, as both agonists and antagonists now exist. My laboratory has focused on understanding physiologic and pathophysiologic determinants of GH sensitivity, which is the integrated effect of GH levels, the abundance of cell surface GHR, and the coupling of GHR activation with intracellular signaling pathways. In previous and the current VA funding cycles, we uncovered a novel mechanism modulating GH responsiveness, involving a proteolytic system composed of the metalloprotease TACE (tumor necrosis factor-a cleaving enzyme) and the TACE inhibitor TIMP3 (tissue inhibitor of metalloproteinases-3) that inducibly alters surface GHR availability by proteolytically shedding the receptor extracellular domain. However, the extent to which this metalloproteolytic system modulates GH signaling in physiological and pathophysiological states is unknown. Realizing that GH levels vary diurnally and with nutritional status, we recently examined whether such factors affect GHR abundance and GH sensitivity. Our preliminary data in mice suggest that hepatic GHR availability and corresponding GH sensitivity vary in a time-of-day-dependent fashion coordinate with the level of TIMP3. Furthermore, diet-induced obesity also affects hepatic GHR abundance. In light of recent reports, we believe understanding how these physiologic and pathophysiologic variables affect GH sensitivity will be crucial to rationally exploit GH-enhancing or -antagonistic therapeutic strategies in critically-ill individuals. In this proposal, we will test two main hypotheses: 1) Hepatic GHR abundance is metalloproteolytically modulated in a time-of-day-dependent fashion. Nutritional factors also affect hepatic GHR abundance, but at a transcriptional level. Both forms of regulation contribute to GH sensitivity and energy homeostasis in the steady state. 2) Critical illness, exemplified by sepsis, acutely desensitizes the liver to GH via TACE- mediated GHR proteolysis. Net effects of time-of-day- and diet-determined GHR abundance and timing and degree of critical illness determine acute hepatic GH responsiveness and may thus impact the course of illness. We will use novel mouse model systems and reagents and techniques we have developed to pursue two specific objectives: 1. Examine in mice potential mechanisms of diurnal variation in liver GHR levels and the impact of such time-of-day effects on hepatic GH sensitivity. 2. Determine how the time of day and feeding regimen impact the propensity of acute illness to modulate hepatic GH sensitivity, using a mouse model of sepsis (lipopolysaccharide (LPS) administration). Successful completion of these studies will definitively ascribe the response of hepatic GH sensitivity to clinically relevant acute illness, enabling rational therapeutic exploitation of the GH axis in sepsis and other forms of critical illness.